Military science fiction is a subgenre of science fiction and military fiction that depicts the use of science fiction technology, including spaceships and weapons, for military purposes and usually principal characters who are members of a military organization, usually during a war; occurring sometimes in outer space or on a different planet or planets. It exists in a range of media, including literature, comics, film, television and video games.

A detailed description of the conflict, belligerents (which may involve extraterrestrials), tactics and weapons used for it, and the role of a military service and the individual members of that military organization form the basis for a typical work of military science fiction. The stories often use features of actual past or current Earth conflicts, with countries being replaced by planets or galaxies with similar characteristics, battleships replaced by space battleships, small arms and artillery replaced by lasers, soldiers replaced by space marines, and certain events changed so the author can extrapolate what might have occurred.

Traditional military values of courage under fire, sense of duty, honor, sacrifice, loyalty, and camaraderie are often emphasized. The action is typically described from the point of view of a soldier in a science fictional setting of or near battle. Typically, the technology is more advanced than that of the present and described in detail. In some stories, however, technology is fairly static, and weapons that would be familiar to present-day soldiers are used, but other aspects of society have changed. Technology may not be emphasized in such stories as much as other aspects of the characters' military lives, cultures, or societies. For example, women may be accepted as equal partners for combat roles, or preferred over men.

When the "extravagan[t]" depictions of war in space operas faded along with pulp fiction more generally, military science fiction developed with a "more disciplined and more realistic notion of the kind of armies which might fight interplanetary and interstellar wars, and the kinds of weapons they might use".

In many stories, the usage or advancement of a specific technology plays a role in advancing the plot, such as deploying a new weapon or spaceship. Some works draw heavy parallels to human history and how a scientific breakthrough or new military doctrine can significantly change how war is fought, the outcome of a battle, and the fortunes of the combatants. Many works explore how human progress, discovery, and suffering affect military doctrine or battle, and how the protagonists and antagonists reflect on and adapt to such changes.

Many authors have either used a galaxy-spanning fictional empire as a background for the story, or have explored the growth and/or decline of such an empire. The capital of a galactic empire is sometimes a "core world," such as a planet relatively near a galaxy's centrally located supermassive black hole, which has advanced considerably in science and technology compared to current human civilization. Characterizations of these empires can vary wildly from malevolent forces that attack sympathetic victims, to apathetic or amoral bureaucracies, to more reasonable entities focused on social progress.

A writer may posit a form of faster-than-light travel in order to facilitate the enormous scale of interstellar war. The long spans of time (e.g., decades or centuries) required for human soldiers to travel interstellar distances, even at relativistic speeds, and the consequences for the characters, is a dilemma examined by authors such as Joe Haldeman and Alastair Reynolds. Other writers such as Larry Niven have created plausible interplanetary conflict based on human colonization of the asteroid belt and outer planets by means of technologies utilizing the laws of physics as currently understood.^{[citation needed]}

Several subsets of military science fiction share characteristics of the space opera subgenre, concentrating on large-scale space battles with futuristic weapons in an interstellar war. Many stories can be considered to be in one or both the military science fiction and space opera subgenres, such as The Sten Chronicles by Allan Cole and Chris Bunch, Ender's Game series by Orson Scott Card, Honorverse by David Weber, Deathstalker by Simon R. Green, and Armor by John Steakley.